The present invention relates to an apparatus for removing liquid from a suspension.
Mixtures of liquids and solids, known as suspensions, present expensive disposal problems to the industries that generate them. Unprocessed suspensions typically cannot be disposed of in landfills due to regulations on water content. Even with more permissive regulations, it is much more expensive to transport and dispose of unprocessed suspensions as compared to solid components because transportation charges and landfill charges correspond to weight.
Additionally, the scope of potential uses of such suspensions is often substantially increased by removal of the liquid component from the solid component. Typically, the value of the dry solids arises from the decrease in weight occasioned by the removal of the liquid fraction, which leads to decreased disposal and transportation costs. The recovered dried solids may also be commercially valuable, such as if they are useable in other industrial and municipal applications (e.g., renewable fuel) or can be sold in secondary markets, such as in the case where the suspensions comprise paper, fiber, coal or mineral slurries.
Unfortunately, efforts to work around the suspension disposal problems often employ methods lacking environmental soundness. For example, many industries dump suspensions, such as waste products, into holding ponds, which are typically large concrete or plastic lined, man-made pools requiring acres of real estate. The suspensions then sit in these holding ponds while the solid materials settle at the bottom over time with the aid of only gravity. Aside from being a slow process, the potential for the pool lining to fail or result in contamination of the surrounding environment makes this a less-than-desirable solution in terms of both efficiency and environmental impact.
Industrial suspension ponds suffer from significant practical difficulties. For instance, holding ponds have a poor resulting yield (dry solid percentage content). Being passive, it also takes a long time to separate water from solids for a given volume of suspension, as compared to devices that rely on active separation. Keeping up with the output for any given suspension flow rate requires a greater area than if active separation systems are used. Two active separation systems, centrifuge processors and belt presses, each produce higher solid content yields than suspension ponds; however, they lack the ability to utilize thermodynamics to achieve 60-100% dry solid percentage yields. These active separation systems are also expensive to purchase and operate, and they are not readily scaled up or down to handle corresponding volumes of industrial suspension flow rates. The lack of portability and limitations on the amount of material which can be processed in a given time are also a significant limiting factor.
Certain drying technologies have been used to further remove water from suspensions processed by belt presses or screw presses. These drying technologies have focused on the use of thermal energy for removing water from the suspensions. For instance, drum dryers have been used; however, the technology is expensive in terms of both capital and operating costs. Belt dryers have also been used and promoted as way of reducing footprint and costs. However, like drum dryers, belt dryers relay primarily on thermal energy to remove the water from a suspension by use of heat. Using heat to remove water requires large amounts of thermal energy to be available, which significantly adds to the operating costs of drying. In addition, both belt and drum dryers lack the ability of flexible throughputs, and also require large systems and high temperatures to operate.
In an attempt to address such limitations and disadvantages in the prior art, in commonly assigned U.S. Pat. No. 8,673,156, which is incorporated herein by reference, a suspension liquid extraction apparatus and method is described. The apparatus is particularly suited for separating water or other liquids from solids in all types of suspensions, including effluents (mixed water and waste) or slurries. In this regard, in the context of U.S. Pat. No. 8,673,156 and in the present application, mixtures of solids and liquids may be referred to as a “suspension.” Such a suspension includes any combination of particulate matter (solids) suspended in or containing significant quantities of water or any other liquid. While it is envisioned that the primary use of the apparatus is to dewater effluents, it is anticipated that the apparatus will be equally useful in any application that requires removal of liquids from any mixture of liquids and solids.
Referring still to U.S. Pat. No. 8,673,156, the apparatus includes at least one arced container for receiving and at least temporarily holding the suspension, which may flow from a suspension discharge pipe or other source onto a conveyor belt which carries a filter. The container may comprise a chamber or compartment bounded by an interior curved sidewall having plurality of graduated, arcuate conduits. A cover (or lid), such as a substantially airtight, preferably non-permeable pliable membrane, may be placed over the container to form an airtight seal within the container. The container may be further divided into a series of chambers using an apparatus that locks into place once the conveyor belt has moved the suspension into the chamber. The apparatus seals around the conveyor belt to create separate chambers within the container.
Each of the chambers is separated by a container seal constructed from a non-permeable substrate to create a dividing wall seal. The suspension will be transferred directly from one chamber to the next while certain liquid removal processes are performed upon the suspension. Preferably, the suspension will be temporarily stored in each chamber for approximately six minutes while the dewatering process is performed. The suspension is moved from the first chamber to the second and subsequently the third chamber using the conveyor belt. The conveyor belt is supported from below to prevent the conveyor belt from significantly contorting during the drying process. Vacuum conditions reaching 23 inches of mercury (77.9 kPa) pressure within a single 6-foot (1.83 m) by 12-foot (3.66 m) chamber can produce over 114,000 lbs (51.2 metric tons) of force from the pliable membrane onto the conveyor belt.
A vacuum is applied to the first chamber within the container through the conduits, and the negative pressure causes the liquid component of the suspension to be forced through the filter and the conduits for recovery. The negative pressure created by the vacuum acts upon the pliable membrane and pulls the membrane towards the vacuum, which, in turn, applies positive pressure on the suspension from above, forcing the liquids of the suspension through a filter. To draw liquid from the suspension in the container, a pressure differential is created across the pliable membrane in each of the chambers to create a squeezing force as the membrane is pulled towards the vacuum.
The graduated design of the sidewall and conduits enable even flow rates. This also results in a substantially equal vacuum pressure being applied to the entire surface area of the filter below the suspension and to the pliable membrane above the suspension and enables captured (filtered) solids in the suspension to be evenly dispersed across the filter media. The pliable membrane acts to absorb the bulk of all vacuum pressure. By doing so, the entire force, over time, of the vacuum is applied to the suspension during the drying process by the pliable membrane. This force creates a negative pressure or vacuum chamber in which water evaporates at lower temperatures. With the additional application of external heat to the chamber, the apparatus can further reduce the water content of the suspension efficiently.
The liquid removal process may occur over multiple chambers to achieve the desired level of dryness for the solid component of the suspension in the container. Additionally, heat may be employed to speed the separation process, and the suspension may also be mixed or agitated for this reason. Heated compressed air injection, radiant heat, pressure, microwaves, and vacuum can be employed to remove the liquid component from the solid component of the suspension in a rapid fashion. Sensors and automatic release valves may be used to ensure that the internal vacuum pressure does not reach an unacceptably high level.
Referring still to U.S. Pat. No. 8,673,156, and characterizing the described invention as a method, the method comprises the steps of: (a) transporting a suspension into a first chamber, the first chamber including a filter restricting access to a drain; (b) sealing the first chamber with a lid, the lid including a pliable membrane contacting the suspension; and (c) applying a vacuum to the first chamber via the drain, whereby the vacuum applies negative pressure to the suspension, such that the negative pressure forces liquid components of the suspension through the filter, and the vacuum applies negative pressure to the pliable membrane, such that the pliable membrane exerts positive pressure against the suspension, such that the positive pressure forces liquid components of the suspension through the filter. The method further includes the additional step (d) applying a rigid cap to the pliable membrane on a side opposite the suspension, the rigid cap exerting pressure on the pliable membrane such that the pressure exerted by the pliable membrane against the suspension is increased. Alternatively, the method may include the additional steps of: (d) unsealing the first chamber; (e) transporting the suspension into a second chamber, the second chamber including a filter restricting access to a drain; (f) sealing the second chamber with a lid, the lid including a pliable membrane contacting the suspension; (g) applying a vacuum to the second chamber via the drain, whereby the vacuum applies negative pressure to the suspension, such that the negative pressure forces liquid components of the suspension through the filter and applies negative pressure to the pliable membrane, such that the pliable membrane exerts positive pressure against the suspension, such that the positive pressure forces liquid components of the suspension through the filter; and (h) applying an airflow of heated compressed air to the suspension, whereby at least a portion of the airflow passes through the suspension such that heat from the airflow and decreased pressure from the vacuum transitions liquid components of the suspension into a vapor phase and expansion of the airflow transports the vapor out of the second chamber via the at least one drain.
However, there remains a need for an alternate apparatus that effectively and efficiently removes liquid from a suspension.